This invention relates to a friction wheel singler for singling sheet material, in particular bank notes, comprising a sheet magazine for receiving a stack of sheets, a singling device with a singling cylinder having one or more friction elements for contacting and conveying a sheet to be singled out of the magazine, and a retaining device forming with the singling cylinder a singling gap through which sheets to be singled out of the magazine are conveyed one by one, the retaining device having one or more friction areas for contacting sheets to be singled out of the magazine.
Friction wheel singlers are used for quickly singling stacks of sheets, for example bundles of bank notes, in the transverse or longitudinal direction so that the singled note can be supplied to a sensor system for determining the authenticity, qualitative nature, value or other characteristic properties of the note.
Friction wheel singlers are based on the principle of a singling cylinder attacking the surface of a note of a bank note stack for example, this specifically contacted note being conveyed in a transport direction due to friction by rotation of the singling cylinder while the other notes of the bank note stack are retained by a retaining device. The retaining device and singling cylinder form for this purpose a singling gap through which the note is conveyed. The retaining device can slightly engage grooves of the singling cylinder, the engagement depth being adjustable. To ensure that the note contacted by the singling cylinder is conveyed and the other notes of the bank note stack retained, a higher force must be exerted on the note in the singling gap by the singling cylinder than by the retaining device on the opposite side of the gap. Therefore, the singling cylinder is usually provided with friction elements whose friction linings have a substantially higher coefficient of friction than the corresponding friction linings of the retaining device, the ratio of coefficients of friction being about 2:1 for example.
It proves to be disadvantageous that the different friction materials of the singling cylinder and the retaining device partly show very different operating characteristics, for example with respect to resistance to environmental influences, moisture absorption, temperature coefficient, aging and wear resistance. This can lead to different service lives and influences the ratio of friction, which can lead to singling errors or even double picks, i.e. more than one sheet being grasped by the singling cylinder.
Singling errors can also occur for other reasons. It is thus usual for the stack of sheets to rest on a supporting plate in the sheet magazine, with feed rolls disposed on a common axle protruding out of the front area (in the sheet transport direction) of the supporting plate in the manner of a hopper. Said feed rolls contact the underside of the lowermost sheet of the stack, thereby lifting the stack from the supporting plate in this area, and convey the lowermost sheet or the stack toward the singling device. Singling gaps occasionally occur when, for example, the sheet material is especially large and the stack of sheets especially heavy (in which case the frictional force between stack and supporting plate is too great), or when the last sheet or sheets to be singled are not, or not completely, grasped by the feed rolls due to a sheet arch.
Singling errors often occur in the singling of stacks of sheets of different kinds and qualities, e.g. bank note stacks with used notes of very different denominations.
GB 2 035 268 A and JP 07 309466 A moreover disclose friction wheel singlers using friction material with approximately the same coefficient of friction for both singling cylinder and retaining cylinder. However, in these friction wheel singlers both singling cylinder and retaining cylinder must be driven. Moreover, the known friction wheel singlers do not permit synchronous singling.